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Original Message
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| RE: Edelbrock Heads and Compression Issue |
| By Gerry Proctor - 12/14/2004 7:32:42 AM; IP 207.133.188.254 |
The displacement of the Edelbrock heads, at 72cc, is within the displacement of the factory combustion chamber(between 70 and 72cc depending on manufacturing tolerance). If you use the same style piston(either dish or flat top depending on the engine), then considering the bore size increase (I think you mean .030, rather than .30 since that would be a wickedly large overbore) your compression would be about the same.
There is a loss of thermal efficiency when going from iron to aluminum heads and the recommendation to maintain that efficiency is to go about a half a point higher on compression. But it's a recommendation you have to take into consideration of the horsepower gains you'll make in the better flowing heads. In other words, you could have better thermal efficiency with a higher compression ratio but that is greatly offset by airflow. If the heads were equal other than the casting material, then you'd loose power but then, that's not the case.
There is also something to be said for a compression ratio a bit below maximum for pump gas. A wider pre-ignition and detonation window means that you have more flexibility in ignition timing. In most cases, an engine that can run 12 to 14 degrees initial will have better driveability than an engine that can only take six degrees. But you also need to pay attention to the cam manufacturer's caution on minimum compression ratios. Using a cam with a minimum compression ratio advisory of 10:1 in a 9:1 engine can make the engine very lazy. You'd want to shoot for high 9s to mid 10s depending on what the cam needs and how you want to use the car. You can go higher but then the engine becomes very sensitive to the tuneup.
So, the heads themselves are not an issue for you. True static compression is and there are plenty of online calculators that will tell you what you're looking at. |
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